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Production of nanocrystalline aluminum alloy powders
through cryogenic milling and consolidation by dynamic
magnetic compaction
by
Umugaba Seminari
A Thesis Submitted to the Faculty of Graduate Studies and Research in
Partial Fulfillment of the Requirements for the Degree of Master of
Engineering
Department of Mining and Materials Engineering
McGili University
Montreal, Canada
August 2007
© Copyright S. Umugaba
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Don't measure yourself by what you have
accomplished, but by what you should have
accomplished with your ability. John Wooden
We are what we repeatedly do. Excellence,
therefore, is not an act, but a habit. Arisfofle
To my family and friends
ABSTRACT
Nanopowders and bulk nanostructred materials have gained large interest in recent
years. Bulk nanostructured materials exhibit properties that are far superior in
comparison to conventional micron grained alloys. The fabrication of large scale
nano-grained materials has been achieved in a two step process: (1) the production of
nanostructured aluminium alloy powders and (2) the consolidation of the powder using
a electromagnetic shockwave process.
The first part consists of cryo-milling; the milling of powder in an attritor filled with liquid
nitrogen. This causes successive welding and fracturing events as the powder is
milled, thereby creating the nano-structure. The low temperature prevents the
possibility of recrystallization and grain growth. The alloy used for this work was AI
5356 (AI-5%Mg). Two different types of raw source materials were investigated: pre
alloyed powders and a mixture of aluminum with pure magnesium or an A1 12Mg17
intermetallic. Experiments have been conducted in order to determine the optimum
milling parameters that will simultaneously give a grain size sm aller than 100 nm;
equiaxed milled particles and mechanically alloyed powder (in the case of the
mixture). The optimum milling parameters were established at 15 hours of milling time
with a rotational speed of 300 RPM and bail to powder weight ratio of 24: 1 in the case
of the pre-alloyed powders. For the mixture of pure aluminum with pure magnesium
the parameters were 15 hours, 300RPM and 32:1. The parameters for the mixture
with the intermetallic were 18 hours, 300RPM and 32: 1.
The dynamic magnetic compaction technique was done with a peak pressure of 1.1
GPa. This ultra-high strain rate process minimizes the exposure of the powders to
high temperature and therefore reduces the possibility of recrystallization and grain
growth. Relative densities of compacted pieces obtained ranged from 86.39% to
97.97%. However consolidation characterized by parti cie to particle bonding with a
melted layer was not accomplished.
ii
RÉSUMÉ
Les poudres nanocristallines et les pièces massives nanostructurées ont gagné en
intérêt ces dernières années. Ces pièces massives nanostruturées ont démontré des
propriétés mécaniques supérieures aux pièces conventionnelles dont la taille de grain
est de l'ordre du micron. L'obtention de larges pièces nanostructurées a été élaborée
en deux étapes: (1) la production de poudres d'alliage d'aluminium nanostructurées
et (2) la consolidation dynamique des poudres ainsi produites en utilisant une
compaction électromagnétique par onde de choc.
La première partie consiste au broyage cryogénique ; les poudres sont broyées dans
un attriteur rempli d'azote liquide. Étant donné la très basse température, il s'ensuit
une succession de soudage et rupture des poudres pendant qu'elles sont broyées et
créant ainsi la structure nanométrique. La basse température permet d'éviter la
recristallisation et de ce fait la croissance de la taille des grains. L'alliage choisi est AI-
5356 (AI-5%Mg). Deux types de source différente de poudres ont été utilisés: une
poudre pré-alliée et un mélange de poudres d'aluminium et magnésium pur ou d'un
intermétallique AI12Mg17. Une série d'expériences ont été conduites afin de
déterminer les paramètres qui influencent le plus les résultats escomptés à savoir la
taille des grains en deçà de 100 nm, des particules dont la forme est équiaxe et la
confirmation de la mécanosynthèse (dans le cas du mélange de poudres). Les
paramètres optimaux de broyage ont été établis à 15 heures de broyage avec une
vitesse de rotation de 300 RPM et un ratio massique de balles sur poudres de 24 : 1
pour la poudre préalliée. Dans le cas du mélange avec le magnésium pur, les
paramètres optimaux ont été fixés à 15 heures, 300 RPM et 32 : 1. Le mélange avec
l'intermétallique a aboutit avec 18 heures, 300 RPM et 32 :.
La consolidation des poudres à l'aide de compaction par ondes électromagnétiques a
été effectuée avec une pression de 1.1 GPa. Cette technique implique un très haut
taux de déformation et minimise l'exposition des poudres à une haute température et
iii
de ce fait réduit la possibilité de recristallisation des poudres. Les densités relatives
obtenues ont été de 86.39 % à 97.97%. La consolidation des poudres, caractérisée
par une couche fondue à l'interface des particules qui assure leur liaison, n'a pas été
réalisée.
iv
ACKNOWLEDGEMENTS
1 would like to extend my sincere gratitude to my thesis supervisor, Prof. Mathieu
Brochu for giving me the opportunity to work toward this project on Nanocrystalline
Production by Cryogenie Milling and Dynamic Consolidation. 1 would also like to thank
him for his unconditional support and technical expertise throughout the course of this
project. In addition, 1 would like to express my gratitude to Alcan, CQRDA, NSERC,
and REGAL for extending financial support to this project.
1 am grateful to Dr Sylvain Pelletier and Patrick England of NRC-IMI, Boucherville, for
providing the access to the dynamic magnetic compaction apparatus and offering
guidance during the consolidation of the pieces. 1 would like to thank Ms. Helen
Campbell, Dr. Kelly Sears, Dr Hojatollah Vali, and Dr Florence Paray for their help
with microscopie characterization. 1 would also like to use this opportunity to thank
Ms. Barbara Hanley, Mr. Raymond Langlois, Mr. Pierre Vermette, Dr. Farzad Jalilian
and Mr. Alain Gagnon for their help in logistic and materials. 1 would express my
sincere gratitude to Mr. Bernie Desjardins for proof reading my thesis work.
And many thanks to my fellow graduate students, especially Sean, Graeme, Ramona,
Andreas, Salim, Phuong, Ehab, Cecile, Yaneth, Paula, Ana Maria, Lydia, Vera, Xin,
Nasser, Dominique, Stella, Abdulaziz, Vikram, Geoff, Geremi, Jinbo, Jessica, Melissa,
Abdel, Curtis, Shawn, Eric and George for their precious help and support during my
master. Last but not the least, 1 would like to extend my sincere gratitude and
appreciation to my family without whose help, support and encouragement 1 would not
have been able to pursue advanced degree here at McGiII University.
v
TABLE OF CONTENTS
Chapter 1 ...................................................................................................................... 1
INTRODUCTION .......................................................................................................... 1
Chapter 2 ...................................................................................................................... 6
LITERATURE REVIEW ................................................................................................ 6
2.1. Nanostructured Materials ............................................................................. 6
2.2. Mechanical Milling ........................................................................................ 8
2.3. Mechanical Alloying .................................................................................... 13
2.3.1. The Process of Mechanical Alloying ................................................. 13
2.3.2. Mechanism of Mechanical Alloying ................................................... 19
2.4. Cryogenie Milling ........................................................................................ 22
2.4.1. Microstructure Evolution and Characterization ................................. 23
2.5. Dynamic Powder Consolidation Techniques ............................................. 31
2.5.1. Shockwave Compaction and Consolidation ..................................... 32
2.6. Mechanical Performance of Consolidated Nanostructured Powders ........ 36
Chapter 3 .................................................................................................................... 39
INSTRUMENTATION AND EXPERIMENTAL SET-UP ............................................ 39
3.1. Flow Chart .................................................................................................. 40
3.2. Materials ..................................................................................................... 41
3.3. Cryomill Setup and Design of Experiment ................................................. 41
3.3.1. Cryomill Setup ................................................................................... 41
3.3.2. Design of Experiments ...................................................................... 42
3.4. X-ray Diffraction .......................................................................................... 45
3.5. Dynamic Magnetic Compaction ................................................................. 49
vi
3.6. Microhardness ............................................................................................ 50
3.7. Microscopie Characterization ..................................................................... 51
3.8. Differentiai Scanning Calorimetry Analyses ............................................... 53
Chapter 4 .................................................................................................................... 54
RESULTS AND DiSCUSSiON ................................................................................... 54
4.1. Microstructural Characterization and Evolution of the Powders ................ 54
4.1.1. Starting Initial Powders ..................................................................... 54
A. SEM and X-Ray Diffraction Analyses ............................................... 55
B. Grain size and Lattice Strain Determination by X-Ray Diffraction of the
Pre-alloyed Powders ........................................................................................ 59
A. SEM and X-Ray Diffraction Analyses .............................................. 61
4.1.3. Characterization of the Cryomilled Powders With the Optimum Milling
Parameters After DOE ..................................................................................... 65
A. SEM and X-Ray Diffraction Analyses ............................................... 66
B. Lattice Constant Determination by X-Ray Diffraction ....................... 69
C. DSC Measurements .......................................................................... 72
D. Grain Size Calculation Based on XRD Compared to TEM
Measurements for the Selected Prealloyed Powders ..................................... 74
4.2. Dynamic Magnetic Consolidation (DMC) ................................................... 78
4.2.1. Optical Microscope Imaging .............................................................. 78
4.2.2. Relative Densities Measurements .................................................... 81
4.2.3 Microhardness and Indent Traces .................................................... 82
4.2.4 Considerations for Full Density Attainment.. ..................................... 86
Chapter 5 .................................................................................................................... 87
CONCLUSiONS ......................................................................................................... 87
vii
REFERENCES ........................................................................................................... 90
viii
Chapter 1
INTRODUCTION
LIST OF FIGURES
Figure 1-1: a) Cryogenie rocket turbopumps with impellers in Ti-5AI-2.3 Sn [10]. b)
Assault amphibious vehicle (AAVA71) using AI-5083 in its hull [11] ................. 3
Chapter 2
LlTERATURE REVIEW
Figure 2-1: Calculated volume fraction of atoms at grain boundaries of a
nanocrystalline sol id as function of grain diameter, assuming that the average
grain boundary thickness ranges from 0.5 to 1.0 nm [17] .................................. 6
Figure 2-2: Schematic skethes of the process of mechanical attrition [6] ..................... 9
Figure 2-3: Arrangement of rotating arms on a shaft in the attrition bail mill. .............. 10
Figure 2-4: Schematic illustration of dislocations evolution and nanometric grain size
formation [20] .................................................................................................... 11
Figure 2-5: Schematic evolution of powder during milling[22] ..................................... 11
Figure 2-6: SEM images of aluminum powders after 2, 4, 6, 8, 10 hours of milling[22]
.......................................................................................................................... 12
Figure 2-7: Mechanical milling parameters[1] .............................................................. 14
Figure 2-8: Change with milling time of crystallite size in mechanically alloyed Cu-Mo
alloys [28] .......................................................................................................... 16
Figure 2-9: Schematic illustration of oxides dispersed in a ductile matrix. The ductile
partiel es form lamella and the oxides are located at the boundaries [1] .......... 21
Figure 2-10 : TEM image showing brittle Er203 particles dispersed in a-titanium
aluminide alloy matrix [1] .................................................................................. 21
Figure 2.11: SEM image showing the embedment of Si in a Ge matrix [1] ................. 22
ix
Figure 2-12: Variation of the time exponent n for isothermal grain growth with the
normalized annealing temperature for nanocrystalline Ni powders[46] ........... 25
Figure 2-13:The DSC scan of the AI-Mg cryogenically milled [48] .............................. 27
Figure 2-14: Grain size and lattice strain versus annealing temperature for two
annealing time [48]. ........................................................................................... 28
Figure 2-15: Lattice constant versus annealing temperature for two annealing time .. 29
Figure 2-16: Nanostructured of a as-milled AI-Mg alloy from a dark filed TEM imaging
with its corresponding diffracted pattern [48] ................................................... 30
Figure 2-17: Distributions of grain size estimated from TEM for an annealed sample of
AI-Mg alloy [48] ................................................................................................. 31
Figure 2-18: Schematic circuit of dynamic magnetic compaction [53] ........................ 32
Figure 2-19: Illustration of the compression mechanism in DMC [52] ......................... 33
Figure 2-20: Schematic representation of two states of condensed matter ahead and
behind a propagating shockwave pulse [54] .................................................... 34
Figure 2-21: Examples of Hugoniot curves for a range of solid metals and alloys [54]
.......................................................................................................................... 36
Figure 2-22: Left: Melted layer surrounding Inconel powders; Right: Inefficient bonding
on Ti3AI particles [56] ....................................................................................... 37
Chapfer3
INSTRUMENTATION AND EXPERIMENTAL SET-UP
Figure 3-1: Cryomill Set-Up ......................................................................................... 42
Figure 3-2: X-Ray Diffraction Apparatus ...................................................................... 46
Figure 3-3: Microhardness Apparatus ......................................................................... 50
Figure 3-4: Optical Microscope Set-Up ........................................................................ 51
Figure 3-5: SEM Apparatus, JEOL 840-A. ................................................................... 52
Figure 3-6: TEM Apparatus JEOL JEM 2010 .............................................................. 52
Figure 3-7: DSC Machine ............................................................................................ 53
x
Chapfer4
RESUL TS AND DISCUSSION
Figure 4-1: SEM images of starting powders ............................................................... 54
Figure 4-2: SEM images and schematic representations of milling parameters for AI-
5356 powders ................................................................................................... 56
Figure 4-3: SEM images and schematic representations of milling parameters for AI-
5356 powders ................................................................................................... 57
Figure 4-4: Diffraction patterns showing braadening of AI 5356 peaks for different
milling set-ups ................................................................................................... 58
Figure 4-5: SEM images and schematic representations of milling parameters of AI
and Mg cryomilled powders .............................................................................. 62
Figure 4-6: SEM images and schematic representations of milling parameters of AI
and Mg cryomilled powders .............................................................................. 63
Figure 4-7: Diffraction patterns showing the evolution of Mg peaks and braadening of
AI peaks for different milling set-ups ................................................................. 64
Figure 4-8: SEM illustrations of morphology of cryomilled powders from the three main
starting sources ................................................................................................. 67
Figure 4-9: Diffraction patterns of different starting powders ....................................... 68
Figure 4-10: Evolution of lattice parameter with increasing atomic percentage of
magnesium[51] ................................................................................................. 71
Figure 4-11: DSC curves obtained fram heating ......................................................... 73
Figure 4-12: TEM images of nanostructured AI 5356: a) Bright field; b) Dark field with
the corresponding diffracted plans .................................................................... 75
Figure 4-13: Grain size distribution based on TEM imaging measures ...................... 76
Figure 4-14: Schematic illustration of samples preparation after DMC ....................... 78
Figure 4-15: Compacted samples ranging fram 100 to 50% of milled powder ........... 79
xi
Figure 4-16: Compacted samples ranging from 50 to 0% of milled powder, plus a
sam pie from flaky powders ............................................................................... 80
Figure 4-17: Evolution of relative density with increasing percentage of milled powders
.......................................................................................................................... 82
Figure 4-18: Indent traces on samples ranging fram 100 to 50% of milled powder .... 84
Figure 4-19: Indent traces on samples ranging fram 50 to 0% of milled powder, plus a
sam pie fram flaky powders ............................................................................... 85
xii
Chapter 1
INTRODUCTION
LIST OF TABLES
Table 1-1: Comparative mechanical properties showing potential of nanostructured
materials .............................................................................................................. 2
Chapter 2
LITERA TURE REVIEW
Table 2-1: Weight capacities of the different types of mills [26] .................................. 15
Chapter3
INSTRUMENTATION AND EXPERIMENTAL SET-UP
Table 3-1: Materials Information: Different sources and characteristic of initial
powders .......................................................................................................... 41
Table 3-2: Detailed conditions of DOE with different levels seL ................................. 44
Table 3-3: Determination of the fourth parameter as a combination of the first three in
the DOE ............................................................................................................ 45
Table 3-4: Siopes and intercepts for grain size and micrastrain calculations[49] ....... 49
Chapter4
RESUL TS AND DISCUSSION
Table 4-1: Grain size calculated values for the pre-alloyed powders .......................... 59
Table 4-2: Values of grain size and lattice strain fram different milling pracess for
comparison ........................................................................................................ 60
Table 4-3: Lattice parameters values of AI and AI alloys under different states .......... 69
Table 4-4: Grain size comparision between XRD and TEM methods ......................... 76
xiii
Table 4-5: Densities and Hardness of Compacted Samples ....................................... 81
xiv
Chapter 1
INTRODUCTION
Research in nanocrystalline materials has increased dramatically in recent years. The
potential of improving the mechanical properties of bulk materials by reducing their
grain to the nanometric scale has been demonstrated by several researchers.
Numerous routes were developed to obtain nanomaterials. Among them, mechanical
milling and alloying (MA) [1], rapid solidification [2], plasma processing [3], vapor
deposition [4], severe plastic deformation processes (SDP) constitute the principal
ones. Severe plastic deformation processes include equal-channel angular pressing
(ECAP), high pressure torsion (HPT), accumulative rolled bonding (ARS), multi
directional forging (MDF), cyclic extrusion and compression (CEC), repetitive
corrugation and straightening (RCS) and twist extrusion (TE). For the present work
the powder metallurgy route has been chosen by means of mechanical milling and
alloying. This route is presented and applied for the production of nanostructured
powders.
Mechanical milling is a solid state process that involves plastic deformation under
shear stress conditions and higher strain rates (101 - 104
S-1) [5]. Lattice defects,
which will yield to high density of dislocations, are the precursors of the nanostructure
formation. They are created during the deformation of the micron scale crystals
composing the powder particles. The dislocations rearrange to form a subgrain
region. The subgrains will be transformed in new grains with a nanometric size.
Mechanical alloying involves a deformation similar to that of mechanical milling. The
difference resides in the formation of an alloy during milling from a mixture of different
metallic powders. Mechanical alloying is a non-equilibrium solid state process that
1
Chapter 1 INTRODUCTION
allows the production of super-saturated sol id solution of metals. The level of
solubility reached is higher compared to the level obtainable from conventional routes
[6].
The alloy chosen in this work is an AI-5% Mg, from the 5XXX series. The
technological importance of this series is related to particular properties such as
corrosion resistance. In addition, Mg (typically <5 wt.%) in AI has the benefit of
increasing strength without compromising ductility [7]. Most important, the alloys
possess a very high strength to weight ratio [8]. shows a comparison of mechanical
properties for a nanostructured AI-5083, a conventional AI-5083 and Ti-5AI-2.5Sn EU.
The n-AI 5083 has a- better specifie ultimate tensile strength (UTS) than the two other
materials. The conventional AI-5083 (AI-4.4Mg-0.7Mn-0.15Cr) is currently used in the
hull of an assault amphibious vehicle (MVA71, see Figure 1-1.) Replacing this
material with an-AI 5083 could save up to 30% of the weight in use. The n-AI 5083
could also replace the Ti-5AI-2.5Sn EU as the material constituting the impellers of the
cryogenie rocket engine turbopumps (see Figure 1-1). This also could save more
weig,ht in the pump due to the lower density of 2.66 for n-AI 5083 compared to 4.48 for
the existing alloy.
Table 1-1: Comparative mechanical properties showing potential of nanostructured materials
UTS Density Spec. UTS Elong. Alloy Temper
MPa g/cc kN.m 1 kg %
Ti-5AI-2.5Sn Annealed 720 4.48 160.7 15
EL! [9]
AI 5083 [9]. H32 320 2.66 120.3 10
AI n-5083 [10] Cryomilled-HIP 462 2.66 173.7 8.4
2
Chapter 1 INTRODUCTION
Figure 1-1: a) Cryogenie rocket turbopumps with impellers in Ti-5AI-2.3 Sn [11]. b) Assault amphibious vehicle (AAVA71) using AI-5083 in its hull [12] .
These two applications are examples of the potential of large scale bulk
nanomaterials.
Mechanical milling will be performed on an AI-5356 alloy (AI-5% Mg). This last alloy
already contains magnesium in solid solution. The formation of an AI-Mg alloy with
the same proportion will be achieved through the mechanical alloying process. During
milling, the tendency of ductile powder particles to adhere to container walls, to
agglomerate, and to sinter to form large millimeter-sized particles has been observed.
This is especially true of the ductile aluminum alloys [13]. Milling in a cryogenie liquid
has been introduced as a solution to eliminate these difficulties. Cryogenie milling, or
cryomilling, is a mechanical attrition process in which powders are milled in slurry
formed with milling balls and a cryogenie liquid. Liquid nitrogen is the cryogenie liquid
most often used.
As the process to fabricate nanostructured alloys is a powder metallurgy process, it
requires a consolidation step. The consolidation of the nanostructure powders can be
3
Chapter 1 INTRODUCTION
achieved following different options. Pressing and sintering, hot isostatic pressing
(HIP) are part of the static process. Shockwave compaction and consolidation is a
viable option because it allows one to maintain the grain size at a nanometer scale
and to achieve high densities in bulk nanostructure materials [14]. The amount of
pressure (over 1GPa) reached during a shockwave can allow consolidation of the
powders and attainment of full density bulk nanomaterials. The loading time, being
very short (-1I..1s), allows the non-equilibrium nanostructured states to be kept [15]. It
is important, at this point, to distinguish between compaction and consolidation. Both
of them can reach high density values, however a compacted sam pie will not have
particle to particle - bonding. A consolidated sample, on the other hand, is
characterized by maximum particle to particle bonding. Therefore, shockwave
compaction and consolidation must be conducted with a careful choice of parameters
in order to realize the bonding between particles. Different methods are used to
produce a shockwave front that will compact the powders. Explosives compaction,
gun compaction and dynamic magnetic compaction are the most common among
them. The process chosen for this work is dynamic magnetic compaction. In this
technique, magnetic fields are used to compact the powder placed in the core of a
coil. High currents are pulsed in the coil to produce magnetic fields in the bore.
Magnetic fields generated induce current in the armature. The armature will interact
with the magnetic fields to produce an inwardly magnetic force on the tube that will
compact the powder. The advantage of this technique relies on the precision of the
forces applied during compaction. In fact, the current produced can be tailored to
generate an exact force on the powders.
The report will present the study in the following order. Chapter 2 presents a brief
literature review on the production of nanostructured materials followed by a
description of dynamic consolidation technique. Topics included are nanostructured
materials definition, process of mechanical milling, mechanism of mechanical alloying
4
Chapter 1 INTRODUCTION
and presentation of cryogenie milling. Efforts are also taken to introduce magnetic
compaction and the evaluation of pieces made from this consolidation method.
Chapter 3 iIIustrates and describes the instrumentation and experimental setup
utilized for producing the nanostructured AI-Mg alloys powders and consolidating them
into bulk pieces. The setup for characterization of microstructures and mechanical
properties are also included.
Chapter 4 presents the experimental results of the work combined with the discussion.
The evolution of the powder with the variation of the milling parameters set by the
design of experiments is presented and explained in this chapter. Microstructural
characterization and selection of the powders for the dynamic compaction are also
presented and finally, mechanical evaluation and optical analyses of the compacted
pieces are executed.
Chapter 5 closes the work with summary and conclusions.
5
Chapter 2
LlTERATURE REVIEW
2.1. Nanostructured Materials
Nanostructured materials are characterized by a m icrostructu rai length in at least one
dimension being up to 100 nm [16]. The main characteristic of a nanostructured
material is the high ratio of atoms in the grain boundary region compared to atoms in
the core of the grain.
100
en
I~ 1.0 nm 1 'i 80 "C \
0.5 nm c: ::::> 0
\ III 60 c: ci! c!i "-.5 40
" cn
"-E
~ 20 ......... <f!-
0 1 10 100
d (nm)
Figure 2-1: Calculated volume fraction of atoms at grain boundaries of a nanocrystalline solid as function of grain diameter, assuming that the average grain boundary thickness ranges from 0.5 to 1.0 nm [16]
This significant difference when compared to conventional materials explains why
nanostructured materials exhibit different physical, mechanical and chemical
properties.
6
Chapter 2 LlTERATURE REVIEW
One should not confuse nanocrystalline materials with nanoparticles, as both
constitute nanomaterials. The former are, as defined in the previous paragraph, a
polycrystalline bulk materials possessing grain sizes sm aller than 100 nm in at least
one dimension, while the latter refers to ultrafine dispersoid particles with diameters
below 100 nm [16].
Structural materials are among the potential applications of bulk nanomaterials since
measured strength are several times higher than for conventional grain size materials.
The Hall-Petch relationship was initially developed for micron-scale grain size
materials and dictates the dependence of yield strength to the grain size, in metals
[17]. As the grain size decreases, an increase in the yield strength occurs, as
expressed in equation 1
Equation 1
Where ay is the yield stress of the material, ao is the friction stress, k is a constant and
d the grain diameter.
Experimental results have shown that the Hall-Petch relationship can be extended for
materials with grain size down to about 20nm. At this critical grain size, nearly 50
vol.% atoms are present at grains boundaries or triple junctions (see Figure 2-1). This
inflection point is called the Hall-Petch breakdown. The breakdown has been
aUributed to different deformation mechanisms that become dominant once the grain
size is reduced down below a critical value [18]. Dislocations pile up is not the major
deformation mechanism due to the fact that grains under the critical value (- 20nm)
cannot sustain a single dislocation within the grain. Therefore, bulk nanocrystalline
materials with grain size ranging between 20 and 100nm will have higher mechanical
properties according to the Hall-Petch relationship. The aUainment of the nanometer-
7
Chapter 2 LlTERATURE REVIEW
scale grains in bulk pieces can be achieved by various approaches which are rapid
solidification [2], plasma processing [3], vapor deposition [4], torsion straining under
high pressure [19] and equal channel angular pressing. The powder metallurgy route
consists of production of nanocrystalline powders and further consolidation into bulk
pieces. This route is more attractive over the other ones due to its capability of
producing powders in large scale and of maintaining the nanometer level after
consolidation. The starting powders can either consist of particles with an average
particle size distribution smaller than 100nm or be nanostructured, meaning they have
a micron scale particle size distribution but possess average grain sizes ranging
between 10-100nm -[20]. Nanostructured powders can be obtained through severe
plastic deformation, such as mechanical milling. The maintaining of the nanometer
grain size at the end of the fabrication process remains the most important challenge
in the production of nanostructured powder.
2.2. Mechanical Milling
Mechanical milling of a metallic powder is a solid state process that involves plastic
deformation under shear stress conditions and high strain rates (101 - 104 s-1)[5].
Lattice defects are created during the deformation of the micron scale crystals of
which the powder particles are composed. Plastic deformation of the powders
originates from the collision with the milling media where some kinetic energy is
transferred, a~ seen in Figure 2-2. Impact energy is related to the momentum of the
milling media. Collision speed of the balls during milling determines the type of milling
which can be separated into two groups, high energy and low energy bail mills.
8
Chapter 2 L1TERATURE REVIEW
lard Steel or "WC" Balls
l Figure 2-2: Schematic sketches of the process of mechanical attrition [5]
Shaker mills can provide high energy milling due to their high frequency and low
amplitudes of vibration. These mills are suitable for small powder quantities (10cm3)
and research purposes [21]. A low energy milling set up can also be used and it
includes attrition mills and tumbler mills. The main advantage of these mills is their
larger milling capacity.
The basic principle of mechanical attrition, as illustrated in Figure 2-3, consists of
powder particles put together with stainless steel or tungsten carbide or ceramic balls
in a container which is shaken or agitated by a rotary shaft. The density of the
grinding medium should be high enough so that the balls create enough impact force
on the powder [1].
9
Chapter 2 LlTERATURE REVIEW
Rota/log impeller
Figure 2-3: Arrangement of rotating arms on a shaft in the attrition bail mill.
As the powders are hit between two milling balls, they are subjected to the successive
steps of flattening, fracturing and cold welding. In the mean time, at a crystalline level,
a nanometer grain size is formed through a three-stage process, as illustrated in
Figure 2-4. It starts with the localization of deformation into shear bands containing a
high dislocation density. The second stage is a combination of annihilation and
recombination of dislocations, resulting in a nanometer-scale sub-grain. As the milling
progresses, the subgrain structure extends throughout the sam pie and the final stage
being the transformation of sub-grain boundary structure to randomly oriented high
angle boundaries [20].
10
Chapter 2
00 LlTERATURE REVIEW
~' .... .'"
$, • . . , .' , ~.. ,~
INCRJ!ASlNG l\-llLLING To.m
============================~~
Figure 2-4: Schematic illustration of dislocations evolution and nanometric grain size formation [20].
Figure 2-5shows the stages schematically and Figure 2-6 depicts Scanning Electron
Microscope (SEM) rnicrographs of the evolution in morphology of pure aluminum
powders cryomilled for different lengths of time [22]. The first stage consists of
flattening particles into flakes. After milling for 2 hours, during stage 2, flakes
formation continues, especially for finer particles and the flakes weld to each other. A
multilayer structure is developed by cold welding. After 4 hours of milling (during
stage 3), the multilayered elongated particles are transformed into a relatively
equiaxed shape. Multilayered flakes fold and reweld without any preferred orientation,
causing a convoluted structure. The layers become convoluted rather than linear. For
milling times of longer than 10 hours, the internai layered structure has disappeared
being replaced by a homogeneous structure and the powder is equiaxed and relatively
spherically shaped [22].
o Stage 0 Stage 1 Stage 2 Stage 3 Stage 4
Figure 2-5: Schematic evolution of powder during milling[22]
11
Chapter 2 LITERA TURE REVIEW
Figure 2-6: SEM images of aluminum powders after 2, 4, 6, 8, 10 hours of milling[22]
Powder characteristics are important in the production of high density compacts. In
the case of cold pressing, aUainment of high green density could be compromised due
to the hardness of the milled powders. An important characteristic is the morphology
of the particles. Hard particles will reach higher green densities, when cold pressed, if
the powders have a spherical shape. Spherical powders tend to have a good
flowability which makes the aUainment of high green density possible. Powders loose
their spherical morphology during milling. The equiaxed morphology is the closest
that powders can be to the spherical shape (see Figure 2-6).
12
Chapter 2 LlTERATURE REVIEW
The next two sections describe two applications of mechanical milling. The first
focuses on mechanical alloying which is a process that enables the production of new
alloys from elemental powders. The second one is cryogenie milling which is a
process in which milling takes place in cryogenie media.
2.3. Mechanical Alloying
2.3.1. The Process of Mechanical AI/oying
Mechanical alloying _ (MA) is a solid-state powder processing technique involving
repeated welding, fracturing, and rewelding of powder particles. This technique has
been used to synthesize equilibrium and metastable phases [1].
The interest and one of the greatest advantages of MA resides in the possibility of
formation of novel alloys that can not be prepared by liquid metallurgy, e.g, alloying of
normally immiscible elements, such as AI-Ta [23] and AI-Nb [24]. This is possible
since MA is fully a solid-state processing technique and therefore limitations imposed
by phase diagrams do not apply [25].
Severa 1 parameters play distinctive roles in MA, they are summarized in Figure 2-7. It
should be noted that the parameters described in Figure 2-7 are not totally
inde pendent.
13
Chapter 2 LITERA TURE REVIEW
Types of mill: High or Low energy
Materials of milling tools: Types of milling media:
Stainless Stee/s or Rods or Balls
Tungsten Carbides or Ceramics
Mechanically 1 Milling 1 Milled Powders Milling temperature
atmosphere 1 1
Milling media-to-powder
1 weight ratio -
l Milling speed J 1 Milling time 1 Process control agent
Figure 2-7: Mechanical milling parameters[1]
The design of a milling experiment has to be done after careful consideration of ail the
parameters.
1. Types of mills and milling media
Milling apparatus can be regrouped into three categories; the following table shows
their respective typical capacities.
14
Chapter 2 LITERA TURE REVIEW
Table 2-1: Weight capacities of the different types of mills [26]
Mill type Sam pie weight
Mixer mills Up to 2 X 20 9
Planetary mills Up to 4 X 250g
Attritors 0.1 -100 kg
2. Milling temperature
Temperature has a crucial role in the mechanical alloying mechanism due to the fact
that atomic solubility varies with temperature. It was found in the case of the
production of AI-Mg alloy, using a Retsch PM 400-MA planetary mill[27] , that
temperature of milling strongly affected the concentration of magnesium in the alloy.
The concentration of magnesium went from 2-3 at. % to almost 25 at% when the
milling temperature was decreased from 70-80 Oc to 20-30 oC. At higher milling
temperatures, the formation of intermetallic phases is favored [27]. This was also the
case in another study, conducted on a Cu-Ag alloy. It was found that the extent of
solid solubility was reported to decrease at higher milling temperatures during
planetary bail milling of a Cu ± 37 at% Ag powder mixture. It was noted that a mixture
of amorphous and crystalline (supersaturated solid solution) phases was obtained
during milling at room temperature while, only a Cu±8at%Ag solid solution was
obtained on milling the powder at 200°C [1].
3. Process control agent
A process control agent (PCA) is used in mechanical milling in order to mediate the
welding of particles. The powder particles indeed get cold-welded to each other,
15
Chapter 2 LITERA TURE REVIEW
especially if they are ductile, due to the heavy plastic deformation experienced during
milling. A balance has to be maintained between cold welding and fracturing of
particles in order to achieve alloying. The PCAs can be solids, liquids, or gases and
they are mostly, but not necessarily, organic compounds [1]. The surface of the
powder particles absorbs the PCA, thus minimizing cold welding between powder
particles. This inhibits agglomeration and promotes fracturing of the particles [1].
4. Milling time
Change in crystallite size, microdeformation, dislocation densities and shape of the
milled particles are affected by the milling time. As an example, in the case of Cu-Mo
alloys, the grain size and particle size decrease with increasing milling time and
approach a minimum value of 20-30 nm. Increasing the milling time beyond this value
results in an increase in the grain size up to 40 nm (Figure 2-8) [28]. This increase
can be due to adynamie recrystallization, grain rotation and growth process
associated with a combination of high local temperature generated during milling and
a large amount of powder with stored energy after severe plastic deformation [28].
100 .1% al. Mo -- e8% al. Mo E 80 e E-CD .e .t::! 60
U)
~ e • 40 }§ • e e e IJ) • ~ 20 • Ü • 0
0 20 40 60 80 100
Milling Ti me (h)
Figure 2-8: Change with milling time of crystallite size in mechanically alloyed Cu-Mo alloys [28]
16
Chapter 2 LlTERATURE REVIEW
5. Milling speed
The energy put into the powder during milling is increased in direct relation to the
milling speed. However, the maximum speed that could be employed encounters
limitations, depending on the design of the mill. In the case of an horizontal mill,
increasing the speed of rotation will also increase the rotational movement of the balls
and above a critical speed, the balls will be pinned to the inner walls of the vial and will
not fall down to impact the powders. Therefore, the maximum speed should be just
below this critical value so that the balls fall down to produce the collision energy [1].
That critical value can be determined experimentally. Calka et al. [29] reported that
when vanadium and carbon powders were milled together at different energy levels,
the final constitution of the powder was different for each level. For example, at a very
low milling speed, the powder consisted of nanometer-sized grains of vanadium and
amorphous carbon, which on annealing formed either V2C or a mixture of V + VC. At
intermediate speed level, the as-milled powder contained a nanostructure, which on
annealing transformed to VC. At the highest speed level, the VC formed directly on
milling [1].
6. Milling media-to-powder weight ratio
The milling media-to-powder weight ratio, also noted as ball-to-powder weight ratio
(BPR) has a significant effect on the time required to achieve a particular phase in the
powder being milled. The higher the BPR, the shorter the time required [1]. The study
of the effect of ball-to-powder weight ratio was elaborated during the milling of an
elemental Ni-AI powder mixture containing 25 at. % of aluminum [30]. It was found
that by decreasing the ball-to-powder weight ratio from 6.5 to 3.25, the start time of the
formation of NbAI phase was delayed from -20 to -40 hour. Similar conclusions were
found in a study on Ti ± 33 at.% AI powder mixture milled in a SPEX mill. The
17
Chapter 2 LITERA TURE REVIEW
formation of an amorphous phase was achieved in 7 hours at a BPR of 10: 1, in 2
hours at a BPR of 50:1 and in 1 hour at a BPR of 100:1 [31]. The number of collisions
per unit time increases at higher BPR because of an increase in the weight proportion
of the balls and consequently more energy is transferred to the powder particles and
so alloying takes place faster [1]. The desired quantity of the powder after milling and
the time doing milling will be the factors determining the optimum BPR.
7. Milling atmosphere
The atmosphere of milling is very important due to the contribution on the
contamination of the powder. The large surface area resulting from small powder
particles size and increasing during milling contributes to the powder's susceptibility to
contamination [25]. In order to avoid such contamination, the powders are milled in
containers that have been either evacuated or filled with an inert gas such as argon or
helium. High-purity argon is preferred in order to prevent oxidation of the powder [1,
32]. Glove boxes with a controlled atmosphere are also used to load and unload the
powder before and after milling. These glove boxes are evacuated and refilled with an
inert gas, usually argon [1]. The use of nitrogen to prevent oxidation has been limited
because of its reactivity with metals, except where the production of nitrides is desired
[1]. The section describing cryogenie milling in liquid nitrogen explains the role of
aluminum nitrides in contributing to an increase in the yield strength and thermal
stability
8. Materials composition of milling tools
They can be made of hardened steel, tool steel, hardened chromium steel, tempered
steel, stainless steels, tungsten carbides or ceramics. Depending of the type of
metals to be milled, the choosing of appropriate media must be given careful
consideration. Milling material is a source of contamination of the powder being
18
Chapter 2 LlTERATURE REVIEW
milled, therefore it is always desirable, whenever possible, to have the grinding vessel
and the grinding medium made of the same material as the powder being milled [1]. A
study on alloying elemental silver and tellurium powder using agate and zirconia as
milling media has resulted in formation of silver silicate (Ag2Si03) and zirconium
telluride (ZrTe2) simultaneously to the Ag2Te phase formation. However, using
tungsten carbide instead of the ceramics for the same powder mixture has not
resulted in the undesired phases [33].
2.3.2. Mechanism of Mechanical Alloying
The mechanism described earlier for mechanical milling also applies for mechanical
alloying. During the milling, the powder partieles are subjected to flattening, cold
welding, fracturing and rewelding, on repeated cycles. In addition here, two or three
types of starting powders are put together in the mill with a given weight percentage.
The powder partieles are plastically deformed by the force of impact of the balls and
the energy input leads to work hardening and fracture. The new surfaces thus created
weld together and form partiel es with an increase in size. The partieles formed have a
composite layered structure containing the starting constituents. Steady-state
equilibrium is reached, after milling for a given period, and it is characterized by a
balance between the rate of welding and fracturing. At this stage of homogenization,
each partiele represents the targeted proportion of ail the original constituents.
Simultaneously, the formation of the nanostructure takes place and the volume
fraction of grain boundary regions also increases, enhancing grain boundary diffusion
mechanism of solute atoms. The increase in temperature during milling favors the
diffusion mechanism and the process of alloying the constituents is established. Even
though mechanical alloying is performed at room temperature, it might be necessary
19
Chapter 2 LITERA TURE REVIEW
to anneal the milled powder to complete the alloying, specially when intermetallic
formation is desired [1].
The three types of material combination that can be created using MA are ductile
ductile, ductile-brittle and brittle-brittle as in the case of metal oxides and
intermetallics.
A. Ductile-Ductile Components
This is the ideal case for mechanical alloying. In order to have plastic deformation that
will result in cold wëlding, a combination of ductile materials is best suited. Ductile
materials also form a layer on the balls surfaces, preventing wear of the milling media
and contamination of the powder [34]. When the homogeneous structure of the
powder is obtained, the completion of the MA process is considered done [1]. A work
on Ni-Cr alloy has shown that it is possible, starting from elemental powders mixture,
to produee an al/oy having the same magnetie behavior as a homogeneous Ni-Cr
alloy produced bya melting procedure [35].
B. Ductile-Brittle Components
The researeh on oxides dispersion strengthening (ODS) falls into this category. The
steps described in the section on the proeess of mechanical milling apply to the ductile
eomponent as weil. While the ductile partiel es are flattened, fractured, and cold
welded, the brittle oxides or intermetallic partiel es get fragmented. The brittle
fragmented partiel es become trapped and oecluded between ductile deformed
particles, as shown in Figure 2-9 [1]. An illustration of the dispersion of Er203 in Q
titanium aluminide alloy created with a Transmission Electron Microscope is shown in
Figure 2-10 [1].
20
Chapter 2 L1TERATURE REVIEW
Figure 2-9: Schematic illustration of oxides dispersed in a ductile matrix. The ductile particles form lame lia and the oxides are located at the boundaries [1]
Figure 2-10 : TEM image showing brittle Er203 particles dispersed in a-titanium aluminide alloy matrix [1].
c. Brittle-Brittle Components
The fact that the two materials are not ductile, precluding cold welding, it could be
assumed that MA will not be achieved. However, mechanical alloying has been
reported for two brittle components, silicon and germanium where a solid solution from
21
Chapter 2 LlTERATURE REVIEW
brittle elemental starting powders (Si and Ge ) was produced after 8 hours of bail
milling with a calculated lattice parameter approximately equal to that of a similar melt
formed alloy [36]. In the case of two brittles components, the particles get fragmented
during milling and their size gets reduced continuously until they reach a limit where
further reduction is not possible, this is called limit of comminution [37]. Like the
embedment of a brittle component in a ductile one, two brittle materials will be.have
the same way with the harder component getting embedded in the less hard
component. Therefore, the harder Si particles are embedded in the less hard Ge
matrix, as shown in Figure 2.11 [1].
Figure 2.11: SEM image showing the embedment of Si in a Ge matrix [1]
2.4. Cryogenie Milling
Milling at cryogenie temperature is done when powders are milled in a slurry formed
with milling balls and a cryogenie liquid such as liquid nitrogen [20]. Cryomilling was
first introduced in the literature for a composite, AI-AI20 3 [38]. [38]. Early research
focused on mechanical alloying of metallic and non-metallic constituents for the
development of dispersion-strengthened alloys for creep resistance improvement [34,
22
Chapter 2 LlTERATURE REVIEW
39, 40]. The organic process control agent (PCA) added to the milling mixture, along
with liquid nitrogen, leads to the formation of oxide (AI20 3), and nitride (AIN)
compounds [41, 42]. These compounds, which are contaminant products or by
products, are useful as dispersoids in the alloys and produce an increase in strength
by acting as dislocation pinning points. Thus, further consolidation of the cryomilled
powder at high temperature would not result in grain growth and the hardness
reached during cryomilling will not be lost. These characteristics allow the creation of
hard homogeneous powder amen able to powder metallurgy (PM) processing when
aluminum powders are used in the cryomilling process [22].
Metals that have a face centered cubic structure, like aluminum and copper base
alloys, stay ductile even at very low temperatures and exhibit plastic deformation [43].
The development of nanostructures is dominated by lattice strain [13], as described in
Figure 2-4. Milling in cryogenie media has been successful for nanostructure
formation of ductile materials, such as aluminum and its alloys. Ductile alloys are
indeed difficult to mill in a non-cryogenie bail mill because of the tendency of powder
particles to adhere to container walls, to agglomerate and to sinter to form large
millimeter-sized particles [13].
2.4.1. Microstructure Evolution and Characterization
A. Grain Growth Kinetics
Thermal stability of materials is related to grain growth mechanisms. In order to
maintain the higher mechanical strength gained by the nanocrystalline structure, the
microstructure evolution under elevated temperature should be understood and if
possible, controlled. Studies on isothermal grain growth kinetics have suggested that
normal grain growth can be expressed as [44, 45]
23
Chapfer 2 LlTERATURE REVIEW
D 2 -D 2 = kt o Equation 2
where 0 is the grain size at time f, Do is the initial grain size and k is the temperature
dependant rate constant. Equation 2 has been unsuccessful in predicting grain
grawth except for high purity metals at high homologous temperatures, which is a ratio
of the actual temperature to the melting point (TIT M) [46]. Therefore, a modification of
the empirical relation was done to obtain better fitting with the experimental results.
The modified equation is represented by Equation 3:
Equation 3
where n (;zV.5) is an empirical time constant determined experimentally and varies
with annealing temperature. Figure 2-12 shows the variation of the exponent n in the
case of nanocrystalline nickel cryomilled powders [46]. This figure demonstrates that
higher temperature is needed to achieve grain growth. Ideally, when the value of n is
0.5 the only driving force on the grain boundary results fram its curvature. However,
deviations fram this value are currents [47] and are attributed to pinning of the grain
boundaries caused by the oxides and nitrides dispersed in the milled powder. This is
the basis of the oxides and nitrides strengthening mechanism described earlier.
24
Chapter 2 LlTERATURE REVIEW
O.!}r------------------,
•
• •
• 0.1'---...1..--........ ---1.------...1..------'
0,4 0,5 0.6 0,7
Normalized Annealing Temperature (TIT M)
Figure 2-12: Variation of the time exponent n for isothermal grain growth with the normalized annealing temperature for nanocrystalline Ni powders [46]
Consequently, the pinning forces of the oxides and the nitrides exerting a drag on the
grain boundaries migration should also be taken into account in the prediction of grain
growth kinetics. As a result, equation 4 was developed to take into consideration the
positive effect of the dispersoids [45].
Equation 4
where Dm is the maximum grain size that results due to the pinning force.
The rate constant k can be expressed, in an Arrhenius form, as follow,
Equation 5
25
Chapter 2 LlTERATURE REVIEW
where Q is the activation energy for grain growth, ka is a constant that is inde pendent
of the absolute temperature T, and R is the molar gas constant [46].
Prior to grain growth, in the annealing process, are recovery and recrystallization [48].
Hence, characterization of grain growth should also imply analyses of the two
preceding mechanisms in order to predict the behavior the cryomilled powder when
subjected to high temperature. The objective is to maintain the grain size below
100nm in order to benefit from the higher mechanical properties related to a
nanometric grain size.
B. Annealing Characterization
Mechanical milling induces large plastic deformation within a material. Therefore, the
material milled contains a large amount of stored energy and when it is subjected to
annealing at elevated temperatures, it will revert to a lower energy state by structural
evolution during recovery and recrystallization [47]. During this annealing process, the
material goes through different phase transformations that can be tracked by enthalpy
changes with Differentiai Scanning Calorimetry (DSC). F. Zhou et al [48] have
analyzed the recovery and recrystallization of a nanocrystalline AI-Mg alloy prepared
by cryogenie milling. The DSC curve (Figure 2-13), performed with a heating rate of
16 Klmin, indicates two distinct exothermic peaks prior to the occurrence of the
melting process (around 530 OC). The first peak is at 164 oC and the second peak at
327 oC. The enthalpy release (llH) was obtained by calculating the area under each
exothermic peak and the values are 450 J/mol, and 410 J/mol, respectively [48].
26
Chapter 2 LITERA TURE REVIEW
(a) HR: 16 Klrnir'
""
• ~ ___ ~~ .... _~ ...... ___ .J--.~ .. _ .....
---..., .~ '. ::1 .. .ci !. 1 v7'C 1 :: 1 Hj4~::' ! 0
1 iI ~ • 1 " 'iiI 1
, <J.I
l-
" J :I: 1,1
a. . 'd ::l 1 g , ,/ 11X) 200 XC 400
/ w
a j'DO 200 ~C<J .. 00 seo SCi} 700
T «q
Figure 2-13:The DSe scan of the AI-Mg cryogenically milled [48]
The exothermic peaks iIIustrate the transformations that occur in the alloy during
annealing. The first peak corresponds to a recovery and the second, to a
recrystallization of the powders [48].
C. Grain Size, Lattice Constant and Lattice Strain Characterization with X-Ray
Diffraction
XRD is often used to calculate grain size in nanostructured materials (5-100 nm) [49].
XRD analyses are limited to the grain size under 100 nm, because the diffraction peak
line broadening that results from a grain ranging from 100 to 300 nm is considered to
be negligibly sm ail [50]. Methods used to estimate grain size with XRD are based on
the broadening of the diffraction peaks, calculated upon the full width half maximum
(FWHM) and the integral width (IntW). Complete descriptions of the methods used to
calculate the grain size are included in the experimental procedure section.
27
Chapter 2 LITERA TURE REVIEW
Along with grain size calculations, XRO is also used to calculate the lattice constant
and lattice strain of the milled powders. The calculated values allow correlating the
grain size, the lattice strain and the lattice constant to the evolution of the ose curve
in order to track phase transformation in the milled alloy. F. Zhou et al [48], in their
study of the microstructure evolution of AI-Mg cryomilled alloy, noted that the grain
size increases from 23 nm in the as-milled powder to a value of 32 nm at the
temperature of the first peak of ose (164°e) and finally to reach 43 nm at 3700 e after
the second peak (see Figure 2-14). This last value indicates the thermal stability of
the alloy. The lattice strain (the root mean square: rms or <e2> 1/2) went from a value of
0.21 ± 0.03% to a value of 0.08% (see Figure 2-14) after the first peak and remained
unchanged after the second peak [48].
w
• 2 min . 45 6 GOmin.
E + -S § 4Qf '" .~ 3-5 ~ '" 1 t :lOr il/ r ~\ < 25l . "T
2Q L.:-"-,-~ .... ~--_..J .. _._. __ l-_-,,"-~_ ••.• _-'----
025r • 2 mm. r 11 60 nnn.
o 2Q~ .'
~ f ~ a 15r ~ ~
i 01û~ 005 ~
l
.. o 00 ! __ .~J.._ .. __ J........-~----L ____ .. j _~ .. _~.l..--..L-_~~_..o.-.-....!_M-'O""~
() 50 100 150 200 250 300 350 400
Annealing tomperatUfe (oC)
Figure 2-14: Grain size and lattice strain versus annealing temperature for two annealing time [48].
28
Chapter 2 LlTERATURE REVIEW
The lattice constant is used to examine the changes in solubility of a solute in the
lattice, in this case of Mg in AI, during the annealing pracess. It was found [48] (see
Figure 2-15) that the lattice constant (ao) decreases fram -0.4078 to 0.4073 nm in the
region of the first peak (150 to 250 OC) corresponding to a decrease of Mg content in
the AI lattice, fram about 7.6 to 5.4 at%. In the region of the second peak (300 to
370°C), an increase of the lattice parameter to 0.4078 nm was found, indicating that
ail of the Mg atoms were again dissolved in the AI lattice. The association between
the lattice constant and the solubility percentage is estimated fram the relationship
between Mg concentration and ao for AI(Mg) solid solutions [51].
4.G8G
4.005
•
• • •
1
~ 1
• 2 min Il 60 min.
1
o 50 100 150 200 250 3{l(l 3-[">0 40{)
Annealing temp€lraturel ('C)
Figure 2-15: Lattice constant versus annealing temperature for two annealing time
29
Chapter 2 LlTERATURE REVIEW
D. Grain Size Characterization with TEM
TEM images analyses is a direct method for grain size measurement. Dark field
imaging shows grain areas of a selected diffraction direction. Despite the fact that
TEM allows one to see the actual image of a grain, it requires more sam pie
preparation and the analysis is done on a small quantity of the powder milled. This
later implies an uncertainty as to the correlation between the sam pie observed and the
total quantity of the powder [49]. Having a high number of measured grains (see
Figure 2-17) and pairing the TEM technique with XRD grain size analyses is
recommended. Figure 2-16 shows an example of TEM image analyses of an AI-Mg
cryomilled alloy [48]. The diffracted planes form ring patterns showing the crystallinity
and absence of a texture in the cryomilled powders. Each concentric circle represents
a diffracted plane of the powder.
Figure 2-16: Nanostructured of a as-milled AI-Mg alloy from a dark field TEM imaging with its corresponding diffracted pattern [48]
30
Chapter 2 LlTERATURE REVIEW
0.5
150 °C/60 min 0.4
--.. -:!< ~ D'" 35.7 nrn c:: 0.3 .Q tî 391 grains ~ "- 0.2 ..8 E ;;, z
0.1
0.0 0 100 200 300 400
Grain size (nm)
Figure 2-17: Distributions of grain size estirnated from TEM for an annealed sample of AI-Mg alloy [48]
2.5. Dynamic Powder Consolidation Techniques
Static routes for consolidation of metal powders into shapes usually consist of
pressing followed by sintering. The two mechanisms can be done in two steps, which
are cold pressing then sintering; or in one step, done by hot pressing or hot isostatic
pressing.
Dynamic consolidation is a viable approach for both maintaining the grain size at a
nanometer scale and achievement of high densities in bulk nanostructured materials
[14]. The choice of dynamic compaction over static is justified by the applied pressure
that can reach over 1 GPa. This high pressure allows achievement of near full density
due to the high energy input and the short duration time (-1I-1s). The short application
time of that high pressure permits maintenance of the non-equilibrium nanostructured
states [15].
31
Chapter 2 LlTERATURE REVIEW
2.5.1. Shockwave Compaction and Consolidation
This principle relies on the generation of a shockwave front that will pass through
green powder compacts in order to consolidate them. It is the energy of the
shockwave that consolidates the powders into dense materials. The densification
occurs at an extremely high strain rate (1 07_1 08S-1) due to pressure levels exceeding 1
GPa imposed in less than 11Js.
Shockwaves can be generated by means of explosives, gas gun or magnetic pulse.
ln this project, the fo~us will be on dynamic magnetic compaction.
A. Dynamic Magnetic Compaction
This technique, also called Electromagnetic Forming (EMF), is based on the
compression of powders within a conductive container which has been placed in the
bore of a coil, by repulsive forces generated by the opposite magnetic field in adjacent
conductors. The primary field is developed by the rapid discharge of a capacitor
through the forming coil and the opposing field results from the eddy current induced
in the armature (Figure 2-18and Figure 2-19). The powders are pressed to full density
via the transmiUed impact energy from the armature, with the entire compaction
occurring within a few microseconds of the compaction cycle [52, 53].
Figure 2-18: Schematic circuit of dynamic magnetic compaction [53]
32
Chapter 2 LlTERATURE REVIEW
Armature Powder
"'Current ... Magnetie flux c:::> Mag netie pressure
Figure 2-19: Illustration of the compression mechanism in DMC [52]
B. Conditions at a Shock Front and Equations of States
A schematic illustration of a propagating shock front is presented in Figure 2-20. In
front of the planar shock wave is the undeformed material and behind it is the
deformed material.
33
Chapter 2 LlTERATURE REVIEW
A ~
Pressure i·"AB
.1 Lltt\
: p 1
1 r
Time
C
Figure 2-20: Schematic representation of two states of condensed matter ahead and behind a propagating shockwave pulse [54]
Equations of states during shockwave consolidation appear as conservations of mass,
momentum and energy. The undisturbed region ahead of the shock front is
characterized by density Po, volume Va, internai energy Eo, pressure Po [54]. The
particle velocity refers to the velocity u that a given element in the material acquires,
as a result of the shock wave passing over the element. The shock velocity is the
velocity U with which the disturbance moves through the body. The undeformed
material to the right of the shock front can be envisioned to move toward the shock
front with a velocity of U-u. Conservation of mass is then written as
peU -u) = PoU Equation 6
34
Chapter 2 LITERA TURE REVIEW
Momentum conservation is written
Equation 7
The shock front applies on the system a pressure equal to P. The pre-existing
pressure is Po. The net force acting on the system is P-Po. The shock wave
accelerates a mass equal to po *U per unit time to the velocity u, which results in a
momentum transfer of po *U*u of the mass element.
Final/y, conservation of energy is written
(E _ E ) = (P + ~) )(Vn - V) o 2 Equation 8
Equations 6 to 8 are referred as the Rankine-Hugoniots equations [54]. By
rearranging these equations, the shock velocity and the particle velocity can be
expressed by:
u= (P-PJ
(Vo - V)
Equation 9
Equation 10
Equations 6 to 10 can lead to the construction of the Rankine-Hugoniot Curve which
represents a two-dimensional curve relating pressure and volume for a material after
the passage of a shock wave. The P-V Hugoniot is a locus of ail the possible end
states that can be achieved behind a single shock wave passing through a material at
a given initial state. The Hugoniot curve is not an equation of state of a material
35
Chapter 2 LlTERATURE REVIEW
because it's a curve containing only one independent variable instead of a surface
(two independent variables) that defines properly an equation of state in a
thermodynamic space.
eo~--~----~-----r----'-----'
50
-• A. <:J ... w CI: 30 j co co U.I a: CI. 20
10
oL---~----~----~----~--~ 0.75 0.80 0.86 O.tO 0.815 1.00
RELATIVE VOLUME (VIVo)
Figure 2-21: Examples of Hugoniot curves for a range of solid
metals and alloys [54]
The path followed by steady shock compression is the chord connecting the initial
state and final shocked state, the" Rayleigh line".
2.6. Mechanical Performance of Consolidated Nanostructured Powders
Mechanical performance of pieces from powder metallurgy consolidation routes relies
deeply on the degree of density attained after consolidation. Legros et al .[55] and
Tellkamp et al [10] experimented with warm compaction and hot isostatic pressing,
36
Chapter 2 LlTERATURE REVIEW
respectively for the attainment of high density and fulfill consolidation of the powders.
Dynamic consolidation techniques have to be used carefully order to achieve full
density and bonding. The process parameters, mostly the impact energy, must be
controlled to assure the bonding between particles. The bonding can be obtained with
a melted layer of the particles during compaction without compromising ail the
nanostructure formed in previous processes. The melted layer guarantees the higher
mechanical properties because the powders are consolidated. When the particles do
not bond with a melted layer, properties rely only on mechanical interlocking, the
powders only having been compacted. This phenomenon has been observed by
Meyers et al [56] where a surrounding layer melted on shockwave consolidated
Inconel 718 powders. Figure 2-22 illustrated examples of shockwave compacted
particles with a surrounding melted layer on Inconel 718 and an inefficient bonding on
T 3AI sample.
Figure 2-22: Left: Melted layer surrounding Ineonel powders; Right: Ineffieient bonding on Ti3AI partiel es [561
The strength of nanostructured metals is often increased at the expense of ductility
[57, 58]. One option to improve ductility is to incorporate larger grains particles to the
nanocrystalline powder at the consolidation stage as Legros et al. [55] and Tellkamp
et al. [10] did with copper and AI 5083 alloy respectively. The tensile tests performed
37
Chapter 2 LITERA TURE REVIEW
on the samples resulted in elongation at fracture of 2.1% and 8.4% respectively, both
of which are higher than with typical nanocrystallinity [59]. In these studies, the
formation of larger grains was achieved by recrystallization during warm compaction
[55] or by bimodal grain growth during hot isostatic pressing (HIP) consolidation and
extrusion [10].
8rochu et al [60] have attempted ta achieve full density and improved ductility by
controlling the shockwave process. Their work aimed to produce pieces where the
grains melted on the surface, producing an ultra-fine layer structure but the core of the
grains maintained the nanostructure characteristic. However an oxide layer present at
the surface of the particles prevented bonding and no liquid film was obtained.
Therefore, the material obtained was made with mechanically interlocked particles.
38
Chapter 3
INSTRUMENTATION AND EXPERIMENTAL SET-UP
ln this chapter, the materials, the procedure and the instruments used are presented
in the order they appear in the chronology of the whole process. The flow chart
summarizes these steps.
39
Chapter 3
3.1. Flow Chart
Startmg Matenals
Expenmental Setup
Execution
Mechanlcal Evaluation
1
1
1
INSTRUMENTAL AND EXPERIMENTAL SET UP
Design of Experiments
Grain Size Lattice Strain
Lattice Constant Mechanical Alloying
40
Grain Size
Chapter3 INSTRUMENTAL AND EXPERIMENTAL SET UP
3.2. Materials
The alloy studied is AI-5356, contains a nominal content of 5 wt % Mg. Two milling
procedures were used, the first one involving solely nanostructure formation (milling
pre-alloyed powders) and the second involving mechanical alloying during
nanostructure formation. Two sources of Mg were used, namely commercial purity Mg
and Ah2Mg17 intermetallic. Table 3-1 presents the characteristics of the starting
powders used. For the two cases where mechanical alloying was involved, the two
powders were carefully weighted to target 5 wt% Mg in the alloy.
Table 3-1: Material$ Information: Different sources and characteristic of initial powders
Powder Supplier Particle size Morphology Purity (Mesh) (%)
AI-5356 pre-VAL/MET -325 Spherical
AI: 94.40 alloyed Mg:5.05
ATLANTIC AI pure EQUIPMENT -325 Spherical 99.9
ENGINEERS Magnesium
ALFAAESAR -325 Spherical 99.8 pure
Ah2Mg17 ATLANTIC AI: 50.05 (Mg50%- EQUIPMENT -200 Angular Mg: 49.70 AI50%) ENGINEERS
3.3. Cryomill Setup and Design of Experiment
3.3.1. Cryomill Setup
The procedure is described as a mechanical attrition technique in which powders are
milled in slurry consisting of mi"ing ba"s and cryogenic liquid. The temperature was
maintained at cryogenic range by contro"ing the lever of liquid nitrogen, which was
monitored by a thermocouple.
41
Chapter3 INSTRUMENTAL AND EXPERIMENTAL SET UP
A Szegvari 01 HD low-energy mechanical attritor (Union Process) with 440C
stainless steels balls (4,76 mm in diameter) was used. Stearic acid
(CH3(CH2)16C02H) was added as process control agent in a proportion of 0.3 wt%.
The container was placed in an enclosed box after milling until completion of liquid
nitrogen evaporation, th en returned to room temperature. The powder was
separated from the milling media by sieving.
Figure 3-1: Cryomill Set-Up
3.3.2. Design of Experiments
Cryomilling of the powder had been performed with the intention of producing
nanostructured pre-alloyed powder on one hand and to obtain mechanically alloyed
aluminum and magnesium nanostructured powder on the other hand. Both of the
routes have to be done with the optimal parameters. A 24-1 fractional plan has been
selected for the cryomill runs in order to determine which parameters most influence
the final powder. This plan totalizes eight different tests for four different parameters,
42
Chapfer3 INSTRUMENTAL AND EXPERIMENTAL SET UP
having each of them vary on two different modalities. After the eight tests, the
preliminary results inform parameter adjustment for further milling. The four milling
parameters that have been chosen are as follows:
• A: Type of powder: Pre-alloyed AI5356 and a mixture of aluminum and
magnesium with the same weight percentage as the alloy which is AI 95 wt%
and Mg 5wt%.
• B: Milling times: 4, 8 and possibly 12 and 16 hours
• C: Rotational speed: 180, 240 and a possibility to increase the speed to 300
RPM (Rotations Per Minute).
• D: Balls-to-powder weight ratio: 24:1 and 32:1
The choice of a fractional plan over complete one allows a reduction in the number of
tests from 16 to 8. The 4th factor becomes a resulting combination of the first three.
Table 3-2 summarizes the conditions of the Design of Experiences.
The expected results will have to conform with the following criteria:
• Reaching the nanometer grain size
• Morphology: Equiaxed particles
• Completion of mechanical alloying in the case of the mixture
43
Chapfer3 INSTRUMENTAL AND EXPERIMENTAL SET UP
Table 3-2: Oetailed conditions of OOE with different levels set
Modalities Identification Labels Ident.
Pre- AI95 wt%- Mg - a
alloyed 5wt%
Powder source
Elemental AI 5356 + A
Mixture
-
Milling Time Short 4 hours - b
Hours Long 8 hours + B
Slow 180 RPM - c
Rotational Speed
High 240 RPM + C
Balls-to-powder Low 24:1 - D
weight ratio High 32:1 + D
44
Chapter3 INSTRUMENTAL AND EXPERIMENTAL SET UP
Table 3-3 shows the combination of the parameters in order to perform a fractional 24-
1 plan.
Table 3-3: Determination of the fourth parameter as a combination of the first three in the DOE
A B C 0 Identification
(A*B*C)
1 - - - - Abcd -
2 + - - + AbcO
3 - + - + aBcO
4 + + - - ABcd
5 - - + + abCO
6 + - + - AbCd
7 - + + - aBCd
8 + + + + ABCO
3.4. X-ray Diffraction
X-ray diffraction (XRO) was used to calculate the grain size, lattice strain and lattice
parameters, so it is possible that it could be used to confirm and quantify the degree of
45
Chapter 3 INSTRUMENTAL AND EXPERIMENTAL SET UP
alloying. The spectra were acquired using a PHILIPS PW1070 diffractometer
equipped with a monochromatic Cu Ka (A=1.54056 A) radiation and operated at
40keV and 20 mA. The angles scanned were from 35 to 120 degrees (29) at a speed
of 0.0100/s (100s/ 0), a dwell time of 0.5 s was used for each step and the step size
was 0.005°. An annealed 5356 powder was used as reference to correct for
instrumental broadening.
Figure 3-2: X-Ray Diffraction Apparatus
The grain size and lattice strain calculations were based on the Williamson-Hall
method [61]. The method, as mentioned earlier, is based on the full width half
maximum (FWHM) and the integral width (IntW) of the diffraction peaks for the milled
and the standard samples, noted f3m , f3a, om and oa respectively. The measured
profile is a convolution of the instrumental broadening (instrument contribution) and
the intrinsic broadening (small grain size and microstrain contribution). Deconvolution
of the measured profile is done on the assumption that the li ne shapes follow a
Gaussian (Equation 11) or Cauchy (Equation 12) approximation or a combination of
the two, Cauchy-Cauchy (CC), Gaussian-Gaussian (GG) and Cauchy-Gaussian (CG)
46
Chapter3 INSTRUMENTAL AND EXPERIMENTAL SET UP
[50]. The best fit between the measured profile and the approximation will decide
which one to use for deconvolution.
Equation 11
Equation 12
ln the deconvolution process, the instrumental profile can be represented by the
measured profile of the annealed sam pie. The representation of the intrinsic and the
instrumental could therefore assume to follow the subsequent combinations [50]:
Equation 13: CC
Equation 14: GG
p2 52 j3 = j3 __ '_1 or 5 = 5 __ a_
m j3 m 5 m m
Equation 15: CG
The overall intrinsic broadening of the milled powders results from small grain size
and microstrain. The deconvolution of the integral intrinsic broadening profile will
consist of separating the integral grain size broadening 00 and the integral microstrain
broadening, os.
Based on the same assumptions as for the instrumental and the intrinsic profiles, the
following equations are applied for the deconvolution of the overall intrinsic profile
[62]:
47
Chapter3 INSTRUMENTAL AND EXPERIMENTAL SET UP
Equation 16: CC
Equation 17: GG
Equation 18: CG
The following equations express the integral grain size and the microstrain broadening
in the milled materia~
JD
= k Dcos8
Js = 4etan8
Equation 19
Equation 20
Where k ranges from 0.9 to 1; " is the wavelength and in the case of CuKa 1, "
=0.154056 nm; 8 is the diffraction angle; D is the grain size; e is an approximate upper
limit of the lattice distortion [50].
Grain size and microstrain can be obtained with the equations obtained when
substituting Equ 19-20 into Equ 16-18, as shown below:
J cos 8 = 4e sin 8 + k D
"cos8 = [ (4esin8)2 + ( ~)2 r'
Equation 21
Equation 22
48
Chapter3 INSTRUMENTAL AND EXPERIMENTAL SET UP
s: B 16 2 sin 2 B k uCOS = e +-
8cosB D Equation 23
ln order to determine the grain size and the microstrain with the previous equations,
the value of 0 has to be calculated first, using Equations 16-18, depending on the
combination (CC, GG, CG) chosen. A plot is do ne with Equations 21-23 and the
slopes and the intercepts of the straight li ne obtained are summarized in the following
table.
Table 3-4: Stopes and intercepts for grain size and microstrain calculations [49]
Combination 1 Plot Siope Intercept
Cauchy-Cauchy 4e kNO
o cos e against sin e
Gaussian-Gaussian
16e2 (kNol ri cos2 e against sin2 e
Cauchy-Gaussian
16e2 kNO o cos e against sin2 elo cos e
3.5. Dynamic Magnetic Compaction
A magnetic press working under a tension of 15.8 kV and generating a total pressure
of 1.1 GPa was used to compact the powders sample. Powders were cold pressed in
49
Chapter3 INSTRUMENTAL AND EXPERIMENTAL SET UP
copper containers to aUain a green density of 65%. A total of 12 samples were
prepared for compaction based on the prealloyed AI 5356. These samples consisted
of a mixture of cryomilled powders and un-milled ones. Starting with a sam pie of
totally cryomilled powders, the percentage of un-milled powders was increased by
10% per sample. An extra sam pie consisting of flaUened powders was also
compacted. Following compaction, samples were cut into two pieces, mounted, and
polished for optical microscopy analyses. The second piece was used to measure the
relative density of each sample. The density was obtained by dividing the measured
weight of the cut piece by the volume calculated with the measured values.
3.6. Microhardness
Microhardness measurements had been conducted on a Clark Microhardness Tester
(CM-100AT) connected to a computer using Clemex Image Analysis Software
(Clemex CMT) for the readings of the measures.
Figure 3-3: Microhardness Apparatus
50
Chapfer3 INSTRUMENTAL AND EXPERIMENTAL SET UP
3.7. Microscopie Characterization
Optical microscopies were done with a Nikon (EPI HOT 200) connected to an
automated Clemex stage (JS 2000) and to a computer using Clemex Vision
Professionnal Edition for the edition of the images.
Figure 3-4: Optical Microscope Set-Up
Scanning electron microscope (JEOL-840-A) was used to characterize the particles
morphology. Transmission electron microscopy observations were done using a
JEOL JEM-201 0 operating at 200 kV.
51
Chapfer3 INSTRUMENTAL AND EXPERIMENTAL SET UP
Figure 3-5: SEM Apparatus. JEOL 840-A
Figure 3-6: TEM Apparatus JEOL JEM 2010
The TEM samples were prepared by spreading the powders on a carbon film.
52
Chapter3 INSTRUMENTAL AND EXPERIMENTAL SET UP
3.8. Differentiai Scanning Calorimetry Analyses
DSe was used to determine through enthalpy evolution, the charaderistics of the
phases present in the powders. The three main powder sources were the prealloyed,
the mixture of pure aluminum and pure magnesium and the mixture of pure aluminum
and the intermetallic. For each sample, 20 mg were used to run the scan. A heating
rate of 10K per minute for a temperature range of 20 to 5000 e was set for the three
samples run.
The apparatus consisted of a High Performance Modular DSe & DT A Thermal
Analyzers, from Setaram, Model Setsys Evolution.
Figure3-7: ose Machine
53
Chapfer4
RESUL TS AND DISCUSSION
4.1. Microstructural Characterization and Evolution of the Powders
4.1.1. Starting Initial Powders
The following scanning electron microscope (SEM) images show the morphology of
each powder prior t.9 milling. The intermetallic AI12Mg17 is the only non spherical
powder and possesses a different initial size, 200 mesh (74IJm) compare to 325 mesh
(44IJm) for ail the remaining powders.
(a) AI 5356 Pre-alloyed (b) Aluminum pure
(c) Magnesium Pure (d) Intermetallic AI12Mg17 (Mg50%- AI50%)
Figure 4-1: SEM images of starting powders
54
Chapfer4 RESUL TS AND DISCUSSION
4.1.2 Characterization of the Cryomilled Powders from the Design of
Experiment Scheme
The characterization of the milled powders, consisting of 8 different setups based on
the design of experiments (DOE), has been separated into two groups. The milling
procedures described earlier (section 3.2), was used to create the two groups (pre
alloyed powder and the elemental mixture). Microstructural characterization is done in
order to confirm the following conditions that the powders have to satisfy:
- Nanometer grain size
- Equiaxed particles morphology
- Completion of the mechanical alloying in the case of the elemental mixture.
4.1.2.1. Microstructural Characterization of Pre-alloyed Powders
A. SEM and X-Ray Diffraction Analyses
The SEM images are paired up with the XRD pattern for a better understanding of the
milling mechanism illustrated by the evolution of particles morphology and peak
broadening. In Figure 4-2 (a), the powder had low milling time (4hours) low rotation
speed (180 RPM) and high ball-to-powder weight ratio (BPR of 32: 1). This is traduced
by flattened, fractured and non-welded particles. Figure 4-2 (b), illustrates equiaxed
particles of powder milled for 8 hours at 180 RPM and BPR of 24: 1. The particles are
in stage 3 of the process of mechanical milling. Figure 4-3 (a) shows powder after 8H
of milling time, 240 RPM and 32:1 for BPR but the particles are not equiaxed. In
stage 2 of the mechanical milling the powders become flattened, fractured and start to
55
Chapter4 RESUL TS AND DISCUSSION
cold weld to each other. The image showed in Figure 4-3 (b) (4H, 240RPM and 24:1
BPR), indicates that the powders are flattened and start to cold weld. A difference on
the edges of the flattened particles of AbCd and the edges of the flattened and
fractured of AbcD has to be exposed.
From this point, one concludes that a higher BPR has the tendency to produce more
fracturing than cold welding. On the other hand, a higher RPM a"ows the powders to
be equiaxed.
(a) AbcD: 4H 180 RPM Ratio 32:1 (b) ABcd: 8H 180 RPM Ratio 24:1
Time (H)
15 12
Ratio
8
4
Time (H)
15 12
8 -/: 1 RPM 4 1 1
1 1
24:1,(1 32y Ratio
Figure 4-2: SEM images and schematic representations of milling parameters for AI-5356 powders
56
RPM
Chapter 4 RESUL TS AND DISCUSSION
(a) ASCD: 8H 240 RPM Ratio 32:1 (b) AbCd: 4H 240 RPM Ratio 24:1
Time (H)
15 12
Ratio
8 4
-----" RPM
Time (H)
15 12
8
Figure 4-3: SEM images and schematic representations of milling parameters for AI-5356 powders
RPM
The XRD patterns illustrated in Figure 4-4 show the broadening of the peaks after
milling powders for the same design of experiment modalities. In order to have an
accurate analysis of this broadening, grain size calculations have been done and are
presented in the next section. However, it can be noted that the lowest spectrum
(AbcD: short time, low rotational speed and high ratio) demonstrated the least
broadening of the peaks.
57
Chapter4 RESUL TS AND DISCUSSION
6000.---------------------------------------------------------------------------------,
5000 -
4000 -~ !: ::J
-ë ~ 3000 ~ III !: $ .5
2000
1000
-AbcD: 4H 180RPM Ratio 32:1 -AScd: 8H 180RPM Ratio 24:1
- ASCD: 8H 240RPM Ratio 32:1 - AbCd: 4H 240RPM Ratio 24:1
o ft!"" ..J r' ....... d' 'L.. J'=
35 40 45 50 55 60
29
65 70 75
Figure 4-4: Diffraction patterns showing broadening of AI 5356 peaks for different milling set-ups
58
80 85
Chapter4 RESUL TS AND DISCUSSION
S. Grain size and Lattice Strain Determination by X-Ray Diffraction of the
Pre-alloyed Powders
Determination of grain size is a direct way of confirming the nanostructured state of
the powder. The method used is based on X-Ray diffraction. This method has been
chosen over the TEM method at this stage in the experiments because it requires less
sam pie preparation and is therefore less time-consuming.
Added to grain size calculations is the lattice strain (microstrain) determination. This
characteristic is alsC? calculated with the X-Ray diffraction patterns using the Hall
Williamson relation. Table 4-1 shows the values of the calculated grain size and the
corresponding lattice strain of the prealloyed powders for different milling setups.
Table 4-1: Grain size calculated values for the pre-alloyed powders
Grain
size (nm) Lattice strain (%)
AbcD: AI-5356 4H 180RPM Ratio 32:1 80 ±29 0.13 ± 0.05
AScd: AI 5356 8H 180 RPM Ratio 24:1 30 ± 11 0.16 ± 0.06
AbCd: AI 5356 4H 240 RPM Ratio 32: 1 120 ± 4 0.31 ± 0.01
ASCO: AI 5356 8H 240 RPM Ratio 32: 1 40 ± 7 0.23 ± 0.04
The value obtained with the 4 hours sam pie (AbCd: 120 ± 4 nm) will not be
considered for the continuation of the work because the grain size obtain is over 100
59
Chapter4 RESUL TS AND DISCUSSION
nm. The grain sizes of the three other samples are ail under 100 nm. These values
satisfy the nanometer grain size criterion. However the morphology of the particles
favors the ASCO sam pie. In fact only this sam pie possesses powders with equiaxed
morphology. This second criteria is only satisfied by the ASCO sam pie.
Consequently, longer milling time and lower (24:1) ball-to-powder weight ratio is the
combination that would be retained at this point of analysis. Table 4-2 shows values
of grain size and lattice strain obtained for cryomilled pure aluminum and AI-7.5% Mg
alloy [13, 63]. The grain size values obtained in the present study are within the same
time range as those fram the studies presented.
Table 4-2: Values of grain size and lattice strain from different milling process for comparison
Milling Grain size LaUice
Sam pie (nm) strain (%) Reference time (h)
AI 8 26 ±2 0.16 ± 0.03 [63]
AI-7.5%Mg 8 26 ±2 0.23 ± 0.03 [13]
AI-5356 40 ±7 0.23 ± 0.04 8 This work
(5%Mg)
60
Chapter 4 RESUL TS AND DISCUSSION
4.1.2.2. Microstructural Characterization of Mixture of Elemental
Powders
A. SEM and X-Ray Diffraction Analyses
Figure 4-5 (a) shows powders in stage 2 of the mechanical milling process and its
corresponding schematic milling parameters representation. The particles were
flatlened, fractured and started to weld and form lamellas. This sam pie (abCD) was
subjected to 4H of milling time, 240 RPM and 32: 1 of ball-to-powder weight ratio
(BPR). Figure 4-5 (b) illustrates particles before the beginning of stage 2, they are
flattened but not fractured and have not started to weld. Thus, 180 RPM and 24:1
BPR requires more time to reach stage 2. The XRD patterns (see Figure 4-7) of these
samples corroborate the SEM pictures analyses. The pure magnesium peak is still
visible, confirming the early stage in mechanical milling process. In the course of the
milling, the magnesium particles will be embedded as the particles are cold welded
and acquire equiaxed morphology (stage 3) in the aluminum matrix. The
homogenization of the powderrs completes the AI-Mg alloy. Figure 4-6 a) shows
images of aBcD powders that have been milled for 8H at 180 RPM and with a BPR of
32: 1. Figure 4-6 b) illustrates of aBCd milled for 8H at 240 RRM and with a BPR of
24: 1. A BPR of 32: 1 introduces more energy and accelerates the alloying process [1]
but 240 RPM plays in favor of more equiaxed particles, as shown in the case of aBCd.
The comparison of the XRD patterns (see Figure 4-74-7) of these samples (aBcD and
aBCd) does not favor one setup over the other one because both of them still show
pure magnesium peaks.
61
Chapter 4 RESUL TS AND DISCUSSION
(a) abCD: 4H 240 RPM Ratio 32:1 (b) abcd: 4H 180 RPM Ratio 24:1
Time (H) Time (H)
15 15 12 12
8 8 RPM
Ratio Ratio
Figure 4-5: SEM images and schematic representations of milling parameters of AI and Mg cryomilled powders.
62
RPM
Chapter4 RESUL TS AND DISCUSSION
(a) aBcD: 8H 180 RPM Ratio 32:1(b) aBCd: 8H 240 RPM Ratio 24:1
lime (H)
15 12
8 4
~
32:1,~'
Ratio ,1
-"/' 1 1
1 1 1 1
1
RPM
lime (H)
15 12
Ratio
8
4
-----7 1 1
1 1 / 1
1
Figure 4-6: SEM images and schematic representations of milling parameters of AI and Mg cryomilled powders.
RPM
At this point, the conclusion that in order to achieve the alloying of the elemental
powders and the equiaxed morphology, it is necessary to mill the elemental mixture
for a much longer time with high RPM and high BPR.
The grain size and the lattice strain of these samples were not calculated because
they did not satisfy mechanical alloying completion condition.
63
~ 'c :1
.ci ... ~ >-:t: t/I C
.Sl
.5
Chapter4
8000
7000
6000
5000 ~ 1 1
4000
3000
O·+-' --
30
• •
0
0
Jill 1\ •
40 50
RESUL TS AND DISCUSSION
- abCD: 4H 240RPM Ratio 32:1 - abcd: 4H 180RPM Ratio 24:1
- aBcD: 8H 180RPM Ratio 32:1 - aBCd: 8H 240RPM Ratio 24:1
• Mg o AI
o o
• • •
60 70 80
26
Figure 4-7: Diffraction patterns showing the evolution of Mg peaks and broadening of AI peaks for different milling set-ups
64
Chapter4 RESUL TS AND DISCUSSION
4.1.3. Characterization of the Cryomilled Powders With the Optimum
Milling Parameters After DOE
After the execution of the 8 tests according to the design of experiment, further tests
were performed with optimized parameters based on the initial conclusion from the
two different types of powder. Therefore, the following results will be presented based
on the following three types of starting powder:
• The pre-alloyed AI 5356
-
• The mixture of pure aluminum and pure magnesium
• The mixture of pure aluminum and the intermetallic AI12Mg17 (Mg50%- AI50%)
The analysis of the results of the DOE has informed new cryomilling parameter levels.
• For the pre-alloyed powder, the previous conclusion suggested long milling
time and lower ball-to-powder weight ratio. In addition, the SEM image
analysis of the two 8-hour samples of the elemental mixture (aBcD and aBCd)
showed that high RPM resulted in more equiaxed particles than lower RPM
(see Figure 4-6). Therefore, the rotation speed has been increased to a value
of 300 RPM, the BPR maintained at 24:1 and for more homogenization, the
milling time has also been increased to 15 hours.
• For the mixture of aluminum and magnesium, milling for a linger time at a high
RPM and BPR has been suggested. Thus, the RPM was increased to 300, the
BPR fixed at 32:1 and the milling time increased to 15 hours.
• The third type of starting powder, mixture of pure aluminum and the
intermetallic had a total milling time of 18 hours, a rotational speed of 300 RPM
65
Chapfer4 RESUL TS AND DISCUSSION
and a ratio of 24: 1. The higher ratio of 32: 1 was not necessary because the
intermetallic powder already forms a phase with aluminum. Therefore the
accomplishment of the solid solution is halfway through. In addition the
intermetallic powder is a hard material; the milling of two materials of different
hardness implies that the harder material will act as a cutter of the softer one.
As a result, formation of finer powder is observed in this particular case.
A. SEM and X-Ray Diffraction Analyses
Optimized paramet~rs produced equiaxed particles for the three types of starting
powders, as shown in Figure 4-8.
The SEM images of the three samples ail show equiaxed particles, suggesting that
the stage 3 in the milling process has been successfully reached. As for the
homogenization (stage 4) of the powders, SEM image analysis is not sufficient as a
tool to confirm the attainment of this stage. Therefore, further analyses are necessary.
The XRD patterns presented in Figure 4-9 are the first stage in the analyses. The
spectra imply that the mechanical alloying has been completed as no pure
magnesium peaks, nor intermetallic peaks are still visible.
66
Chapter4 RESUL TS AND DISCUSSION
(a) 535615 H 300RPM Ratio 24:1 (b) AI+Mg Pure 15H 300RPM Ratio 32:1
(cyAI + A1 12Mg17 (Mg50%- AI50%) 18H 300RPM Ratio 24:1
Figure 4-8: SEM illustrations of morphology of cryomilled powders from the three main starting sources.
67
(il
5000
4500
4000
3500
~ 3000 ::1
€ ~ 2500 ~ //1
ai 2000 -.E
1500
10001
500
o 35
Chapter4
40 45 50 55
RESUL TS AND DISCUSSION
- AI 5356 15H 300 RPM Ratio 24:1
-AI pure + Mg pure 15H 30Ç RPM Ratio 32:1
- AI pure + Intermetallic 18H 300 RPM Ratio 24:1
60
29
65 70 75 80
Figure 4-9: Diffraction patterns of different starting powders
68
l
85
Chapter4 RESUL TS AND DISCUSSION
B. Lattice Constant Determination by X-Ray Diffraction
ln order to confirm the mechanical alloying as stated by XRD patterns, lattice
parameters determination is a powerful tool. Calculations of the lattice parameters for
the three cryomilled powders will permit a clear comparison between the prealloyed
powders and the powders obtained fram a mixture of pure aluminum and pure
magnesium on one hand and pure aluminum and an intermetallic on the other hand.
The assertion is based upon a relation between the percentage attained of sol id
solution of magnesium in the aluminum matrix and the corresponding lattice
parameter of the aluminum alloy [51]. This assumes that the lattice expansion can be
attributed solely to the dissolved Mg atoms. For this purpose, the program X-LAT
[64](Least Squares Refinement of Cell Constants) has been used for the
determination of the different lattice parameters.
Table 4-3: LaUice parameters values of AI and AI alloys under different states
Lattice parameters
(Angstroms)
AI pure 4.0495 [51]
AI 5356-0 4.0678 [51]
AI 5356 15H 300RPM Ratio 24:1 4.0640 ± 0.0007
AI pure + Mg pure 15H 300RPM Ratio 32: 1 4.0508 ± 0.0007
AI pure + Intermetallic 18H 300RPM Ratio 24:1 4.0508 ± 0.0008
69
Chapter4 RESUL TS AND DISCUSSION
Despite the fact that the X-Ray spectrum show a complete disappearance of pure
magnesium and intermetallic peaks, the values of calculated lattice parameters
demonstrate that the alloying was incomplete. The two samples from a mixture of
pure aluminum and magnesium show the same lattice parameter value of 4.0508 A. Figure 3-10 depicts the evolution of lattice parameters with increasing atomic
percentage of solid solution of magnesium in aluminum. A linear tendency curve has
been added for the equivalent weight percentage. From this tendency curve, the
amount of Mg that has formed a solid solution with aluminum, for both sources of
magnesium, was calculated to be 0.6259 wt%. Note that the equilibrium solubility of -
Mg in AI is only 1.08 wt. % at room temperature [65].
70
Chapter4 RESUL TS AND DISCUSSION
• [51EII] • [50Dor] [51 Poo] • [62Hel] " [64Luo] • Weight% - Linear (Weight %) i
0.424 -,--..... _--... 1
1
0.422 i---"·' . --------------
.. ".L·_·······I 0.420 +-- -----.-" ... -.".-...... ".".----." .-.. 1
0.418 1'- .-------.-. ------ .. __ --'L.~. -1 I°.416 " ~ --- --------~------ --
m "
1
~ 0.414 1
----- -------------_. ----~ ID E "
1
~ If. 0.412 ---------------- .~"
ID • () 1 'e
1 j 0.410 -..-c' ._---"--,,
• 0.408j---· ... --Jl~ '--1 0.406 1
. .,- ,, ___ o.
1
-1
1 1
0.404 +- ..... ----.---." .. _-_.~
0.402 '
0 5 10 15 20 25 30 35 40 45
Atomic Percent Magnesium
Figure 3-10: Evolution of lattice parameter with increasing atomic percentage of magnesium[51]
71
Chapter4 RESUL TS AND DISCUSSION
C. DSC Measurements
One other way to determine the state of the powder milled is to measure the enthalpy
released through a Differentiai Scanning Calorimetry. This technique allows an
evaluation of a compound's change in enthalpy and links it with a modification in the
phases constituting the material. In this case, the liberation of magnesium from the
solid solution with aluminum is the phase transformation that has been aimed at [48].
For the prealloyed AI 5356, the phase transformation happened at 375°C and was
related to a change of enthalpy of 28.2506 J/g. The samples fabricated from
mechanical milling of pure aluminum and magnesium/ intermetallic did not have a
change in enthalpy, as observed in Figure 4-11.
72
Chapter4 RESUL TS AND DISCUSSION
25 .
- AI 5356 - AI pure + Mg pure - AI pure + Intermetallic
20
15
§' 10 .§.
~ 0 Li: -CIl 5 <II :I:
0 6, 100/ // 200 300 400
~ 500
-5
-10
Temperature (oC)
Figure 4-11: Dse curves obtained fram heating
73
Chapter4 RESUL TS AND DISCUSSION
D. Grain Size Calculation Based on XRD Compared to TEM
Measurements for the Selected Prealloyed Powders.
As per the results presented in the previous sections, the analyses were continued
only for the prealloyed powder (15H 300 RPM Ratio 24: 1) in this work. These
powders were chosen for the attainment of the nanometric scale, the formation of
equiaxed particles (better flowability) and the fact that they contain Mg in sol id
solution.
TEM images, shown_ in Figure 4-12, were taken fram cryomilled particles thin enough
to let the electron beam pass thraugh them. Most of the parti cl es used had size under
11..lm; the bright field image was (Figure 4-12 a) done with a particle -0.7I..lm in length.
These particles will have a smaller grain size than of the average of ail the cryomilled
powder grains. Similar results were found by G.Lucadamo et al.[66] where the
particles analyzed had -0.5I..lm in size and possessed grain sizes of -10 nm.
The dark field image (Figure 4-12b) was formed fram a selected plane of the
diffraction pattern rings. The rings in the diffraction pattern confirm the polycrystalline
nature of the cryomilled alloy. The limited number of diffracted planes resulted in a
series of dots in a circle pattern instead of full circles. These dots could suggest a
texture in the cryomilled powders.
74
Chapfer4
lilIIJ·'bI
RESUL TS AND DISCUSSION
a
Figure 4-12: TEM images of nanostructured AI 5356: a) Bright field; b) Dark field with the corresponding diffracted plans
75
Chapfer4 RESUL TS AND DISCUSSION
For the TEM measures, a total number of 158 grains has been used to directly
measure on the image the crystallite size. A distribution of the measured grain size is
shown in Figure 4-13.
60 ,------------------------------------------------,
50
- 40 e c o ~ .ê 30 ... QI
J:I E :::1
Z 20
10
o.
0·-10 10--20 30--40 Grain size (nm)
D=19.6 nm 158 grains
30--40
Figure 4-13: Grain size distribution based on TEM imaging measures
40--50
The grain size obtained from the two methods is summerized in Table 4-4 ..
Table 4-4: Grain size comparision between XRD and TEM methods
Sample X-Ray Diffraction TEM measures
Grain size (nm) 48 ± 3 19.6 ± 6.0
76
Chapter4 RESUL TS AND DISCUSSION
The average value obtained from the TEM is sm aller than the one from the XRD
technique. This is explained by the fact that the particles used for TEM imaging had,
as mentioned earlier, to be small enough to let the electron beam cross them. This
implies that sm ail and thinner particles will also consist of grains with sm aller diameter
size. On the other hand, XRD is done on a much bigger sam pie size and the grain
size obtained is an average value from particles beUer representing the cryomilled
powder.
77
Chapfer4 RESUL TS AND DISCUSSION
4.2. Dynamic Magnetic Consolidation (DMC)
Dynamic magnetic compaction (DMC) of the powders was done by mixing cryomilled
with an increasing quantity of non milled powder having larger grains. The selected
powders for DMC were the prealloyed AI 5356 (15 hours 300RPM, Ratio 24:1).
4.2.1. Optical Microscope Imaging
The compacted samples have been eut in the longitudinal direction and then mounted,
as iIIustrated in Figure 4-14.
~-1 1
1
1
Figure 4-14: Schematic illustration of samples preparation after DMC
The longitudinal faces of the compacted pieces are presented in Figure4-15 and
Figure 4-16. The increasing percentage of non milled powders, is illustrated by the
presence of bigger and clearer (white) particles. The cryomilled particles consist of
darker (grey) particles. The black spots in the indicate porosity, due to a lack of
complete compaction. The last sample was compacted with flaUened-fracture
powders (flaky powders). The curvature of the compacted particles is indicative of
deformation during compaction.
78
Chapfer4 RESUL TS AND DISCUSSION
100% Milled powder 90% Milled Powder
80% Milled powder 70% Milled Powder
60% Milled Powder 50% Milled Powder
Figure 4-15: Compacted samples ranging from 100 to 50% of milled powder
79
Chapfer4 RESUL TS AND DISCUSSION
40% Milled Powder 30% Milled Powder
20% Milled Powder 10% Milled Powder
0% Milled Powder Flaky Powder
Figure 4-16: Compacted samples ranging from 50 to 0% of milled powder, plus a sample from flaky powders
80
Chapter4 RESUL TS AND DISCUSSION
4.2.2. Relative Densifies Measuremenfs
Table 4-5 shows the values of the measured densities for the different combinations of
compacted samples. The measures had been done by weighting eut samples of
calculated volumes. Theoretical density fram which the relative densities are
compared is the density of AI 5356, which is 2.64g/cc.
Table 4-5: Densities and Hardness of Compacted Samples
D~creasing P prel percentage of milled
powders. (g/cc) (%)
100% 2.28 ± 0.03 86.39 ± 5.88
90% 2.31 ± 0.01 87.49 ± 2.42
80% 2.43 ± 0.01 91.89 ± 2.22
70% 2.41 ± 0.03 91.37 ± 6.33
60% 2.46 ± 0.01 92.93 ± 1.94
50% 2.46 ± 0.01 93.01 ± 1.09
40% 2.50 ± 0.01 94.73 ± 2.56
30% 2.48 ± 0.03 93.90 ± 5.49
20% 2.44 ± 0.01 92.48 ± 1.78
10% 2.59 ± 0.01 97.97 ± 2.03
81
Chapfer4 RESUL TS AND DISCUSSION
The values of the relative density can be related to the pictures of the compacted
samples (see Figure4-15 and Figure 4-16). Figure 4-17 shows a tendency of a
decrease in relative density while the milled powder percentage increases. This
tendency is also visible in the pictures of the compacted samples where the porosity
area (black spots) decreases with decreasing percentage of milled powders
105,00·,-----------------------------,
100,00 .
l ~ ~
95,00·
1 ijj
1 l 1
c CIl C CIl .~ 'lii 90,00 (jj
! Il:::
85,00·
80,00 +--------,---,------,------,------,--------,---,------,----1
0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 110%
Percentage of Milled Powders
Figure 4-17: Evolution of relative density with increasing percentage of milled powders
4.2.3 Microhardness and Indent Traces
An indication of how much the powders are consolidated is the indent load mark after
microhardness measurements. In fact consolidation is characterized by a bonding
layer between particles. Compaction, on the other hand, is characterized by a
82
Chapter4 RESUL TS AND DISCUSSION
mechanical interlocking of powders without. An apparent crack at the corner of the
load mark indicates an unconsolidated powder. The images shown in Figure 4-18
and Figure 4-19 indicate a lack of bonding between particles as it appears that ail of
them possess a crack, even if very small for some of the samples, at one of the
corners of the indent trace. Therefore, complete consolidation was not achieved
during the dynamic magnetic process.
83
Chapter4 RESUL TS AND DISCUSSION
100% Milled Powder 90% Milled Powder
80% Milled Powder 70% Milled Powder
60% Milled Powder 50% Milled Powder
Figure 4-18: Indent traces on samples ranging fram 100 to 50% of milled powder
84
Chapfer4 RESUL TS AND DISCUSSION
40% Milled Powder 30% Milled Powder
20% Milled Powder 10% Milled Powder
0% Milled Powder Flaky Milled Powder
Figure 4-19: Indent traces on samples ranging fram 50 to 0% of milled powder, plus a sample fram flaky powders
85
Chapfer4 RESUL TS AND DISCUSSION
4.2.4 Considerations for Full Density Attainment
The peak pressure reached during compaction was 1.1 GPa. Ali samples were
prepared prior to compaction with a green density of 65%. After dynamic compaction,
none of the samples reached maximum density (100%). Shockwave peak pressure
applied during compression generates energy that was dissipated into void collapse,
defect and microkinetic energies [56]. The void collapse energy is affected by partiele
geometry, partiele contact areas and mechanical properties of the particle and
adjacent partieles [56]. Partiele characteristics such as hardness, size, distribution
and aspect ratio will-have a direct effect on the void collapse energy. Defect energy
originated from energy deposited into the interiors of the partieles by generating high
dislocation densities [56]. Microkinetic energy consists of the plastic flow of the
materialleading to an interparticle impact [56]. Consolidation of powders occurs when
the surplus of shock energy dissipates through reaction bonding energy and melting
energy. This could be achieved by increasing the shockwave energy to a higher level,
allowing a formation of melted layer on the partieles that will enhance the bonding [56]
and simultaneously increasing the final density. Dynamic magnetic compaction
(DMC) with its ease of adjusting the pressure is a powerful tool to realize both
compaction and consolidation of powders.
86
Chapter5
CONCLUSIONS
The production of nanostructured AI-Mg alloy powders was processed via the
cryomilling procedure. Two options were available for the formation of AI-5%Mg
nanocrystalline alloy: Milling of a pre-alloyed AI-5356 powder and attempted
mechanical alloying of AI and Mg. Two potential sources of Mg were pure magnesium
and an intermetallic AI12Mg17 powder. Consolidation of the nanostructured powders
was performed with a dynamic magnetic compaction technique. The conclusions are
summarized below.
1. Design of experiment has been conducted and the optimum milling parameters
for the formation of nanostructure pre-alloyed powder were determined. It was
found that 300 RPM (rotational speed), a ball-to-powder weight ratio of 24:1
and milling time of 15 hours were the optimum milling parameters for the
attainment of a nanocrystalline structure and equiaxed powder morphology.
2. In the case of mixture of elemental AI and Mg. The milling conditions were
fixed at 300 RPM, a ratio of 32:1 and a milling time of 15 hours. The
confirmation of the alloying could not be assured despite the disappearance of
the pure Mg peaks in the X-ray diffraction (XRD) spectra. Differentiai scanning
calorimetry (DSe) and lattice constant calculations revealed the lack of alloying
formation during mechanical milling. No enthalpy variation was found during
heating and the lattice constant obtained has the same value as the pure AI,
4.050 A.
87
Chapter5 CONCLUSIONS
3. The second source of Mg, the intermetallic A1 12Mg17 powder, was milled with
pure aluminum at 300 RPM, having a ratio of 24:1 for a total time of 18 hours.
As for the pure magnesium powder, alloying was not confirmed, either by OSC,
or the lattice constant. The enthalpy variation with OSC measurement did not
change and the lattice constant found was also equal to 4.050 A.
4. Analyses were profoundly conducted for the pre-alloyed nanostructured AI-
5356 powder. The grain size obtained with the XRO method was 48.3 nm and
the measure obtained with TEM imaging analysis was 19.6 nm. The difference -
in the values resides in the particles selected for TEM. They were small and
thin, in order to let the electron beam cross them.
5. OSC measurement of the pre-alloyed AI-5356 powders reveals an enthalpy
variation of 28.2506 J/g at 375°C.
6. Oynamic magnetic compaction was done on the pre-alloyed nanostructured AI-
5356 powders. Samples were set by mixing the powder with an increased
percentage of un-milled AI-5356 powders, for better ductility. The resulting
formed pieces attained relative density ranging from 86.39 to 97.97%. The
lowest value came from the 100% milled powder sam pie and the highest from
the mixture of 90% unmilled and 10% milled powders. Oespite that relatively
high density, particle to particle consolidation was not successfully achieved.
7. General conclusions:
a. Cryogenie milling has successfully produced a nanometric grain size in
AI-5356 powder alloys having equiaxed particle morphology. In
addition, milling parameters were optimized for further purposes.
88
Chapter5 CONCLUSIONS
b. Dynamic magnetic compaction was found to be a precise technique for
powder compaction. However, in this work, pressure obtained (1.1
GPa) was not sufficient to ensure consolidation of the powders.
c. In order to have an evaluation of the mechanical properties of the
dynamically compacted samples, longer specimen will have to be
prepared in future work. These samples will have to be long enough so
that tensile testing could be carried out following standard size
specimens.
89
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